Terephthalic acid is commercially produced by oxidation of paraxylene in the presence of a catalyst, such as, for example, Co, Mn, Br and a solvent. Terephthalic acid used in the production of polyester fibers, films, and resins must be further treated to remove impurities formed as a result of the oxidation of paraxylene.
Terephthalic acid (TPA) is an intermediate in the production of polyesters for plastics and fiber applications. Commercial processes for the manufacture of TPA are often based on the heavy-metal catalyzed oxidation of p-xylene, generally with a bromide promoter in an acetic acid solvent. Due to the limited solubility of TPA in acetic acid under practical oxidation conditions, a slurry of TPA crystals is usually formed in the oxidation reactor. Typically, the TPA oxidizer slurry is withdrawn from the reactor, and TPA solids are separated from the oxidizer mother liquor using conventional solid-liquid separation techniques. The oxidizer mother liquor, which contains most of the catalyst and promoter used in the process, is recycled to the oxidation reactor. Aside from the catalyst and promoter, the oxidizer mother liquor stream also contains dissolved TPA and many by-products and impurities. These by-products and impurities arise partially from minor impurities present in the p-xylene feed stream. Other impurities arise due to the incomplete oxidation of p-xylene resulting in partially oxidized products. Still other by-products result from competing side reactions formed as a result of the oxidation of p-xylene to terephthalic acid.
The TPA solids undergo a solid-liquid separation wherein fresh solvent is utilitized to displace a major portion of the liquid component of the oxidizer mother liquor. After drying, the TPA solids are contaminated with impurities that were present in the oxidizer mother liquor since these impurities may be incorporated into the TPA solids. Impurities are also present due to occlusions in the TPA crystal structure and due to incomplete removal of the oxidizer mother liquor by the fresh solvent wash.
Many of the impurities in the oxidizer mother liquor stream that are recycled are relatively inert to further oxidation. Such impurities include, for example, isophthalic acid, phthalic acid and trimellitic acid. Impurities, which may undergo further oxidation are also present, such as, for example, 4-carboxybenzaldehyde, p-toluic acid and p-tolualdehyde. Oxidation inert impurities tend to accumulate in the oxidizer mother liquor upon recycle. The concentration of these inert impurities will increase in the oxidizer mother liquor until an equilibria is reached whereby the rate of removal of each impurity via the TPA product balances with the rate of formation and the rate of addition to the oxidation process. The normal level of impurities in commercial crude TPA makes it unsuitable for direct use in most polymer applications.
Conventionally, crude TPA has been purified either by conversion of a dimethyl ester or by dissolution in water with subsequent hydrogenation over standard hydrogenation catalysts. More recently, secondary oxidative treatments have been used to produce polymer-grade TPA. It is desirable to minimize the concentration of impurities in the mother liquor and thereby facilitate subsequent purification of TPA. In some cases, it is not possible to produce a purified, polymer-grade TPA unless some means for removing impurities from the oxidizer mother liquor stream are utilized.
One technique for impurity removal from a recycle stream commonly used in the chemical processing industry is to draw out or “purge” some portion of the recycle stream. Typically, the purge stream is simply disposed of or, if economically justified, subjected to various treatments to remove undesired impurities while recovering valuable components. One example of this process is U.S. Pat. No. 4,939,297 herein incorporated by reference in its entirety to the extent it does contradict statements herein. The amount of purge required for control of impurities is process-dependent; however, a purge amount equal to 10-40 wt % of the total oxidizer mother liquor stream is usually sufficient to produce TPA adequate as feedstock for commercial polymer manufacture. In the production of TPA, the percentage of the oxidizer mother liquor stream purge necessary to maintain acceptable impurity concentrations, coupled with the economic value of the metal catalyst and solvent components in the oxidizer purge stream, make simple disposal of the oxidizer purge stream economically unattractive. Thus, there is a need for a process that recovers essentially all of the valuable metal catalysts and acetic acid contained in the oxidizer purge stream while removing a major portion of the impurities present in the oxidizer purge stream. The metal catalyst can be recovered in an active form suitable for reuse by direct recycling to the p-xylene oxidation step.
One benefit of this invention is the energy and capital cost savings compared with the extraction based purge process previously described.
Another benefit of this invention is its efficacy compared with extraction purge processes regarding the usefulness of the solvent stream(s) recycled to the TPA process. The primary motivation in a liquid extraction process is based upon the assumption that introducing any aromatic impurities into a p-xylene oxidation process for producing terephthalic acid has a detrimental effect on the terephthalic acid powder quality (e.g. yellow color). Hence, it was assumed that a broad spectrum removal of aromatic impurities, such as provided by liquid extraction, was necessary to achieve appropriate terephthalic acid powder quality.
In one embodiment of this invention, however, employs a relatively simple process that separates benzoic acid from an aqueous solvent. The efficiency of the process toward benzoic acid is high since benzoic acid is more volatile (a higher vapor pressure) than most identified aromatic impurities in the production of a carboxylic acid, typically terephthalic acid. These aromatic impurities include, but are not limited to, trimellitic acid, isophthalic acid, stilbenes, and anthraquinones. Therefore, it is rather surprising that removal of a benzoic acid in favor of the other known impurities, that are inherently colored, would be sufficient to produce a carboxylic acid, typically terephthalic acid of good quality.